The present invention relates to construction of an electrochromic display (hereinafter referred to as "ECD") containing a material of which the light absorption properties are reversibly varied in response to supply of the current, and an electrolyte. More particularly, the invention is concerned with an improvement in an ECD cell with a sandwich structure consisting of two support plates at least one of which is transparent.
It is well known that the electrolyte using ECD cells are classified into two groups. For example, there are one group using inorganic solid state thin layers (U.S. Pat. No. 2,319,765--Talmer, 1943) and another group using organic compounds such as halogens and halogen salts (U.S. Pat. No. 3,854,794-H. T. Van Dam et al., 1974). It was reported that the EC phenomenon in the former is such that, when the thin film is supplied with a negative potential with respect to the electrolyte, the formation of tungsten bronze is carried out by ion-electron double injections to obtain coloration and, when supplied with a positive potential with respect to the electrolyte, reactions opposite to the above are carried out to perform bleaching (B. W. Faughnan et al., RCA Rev. 36 (4), 177, 1975). In the latter, electrolytic reduction of halogen ions develops radical cations or radical ions so that coexisting halogen ions and salts are formed and deposited on the surfaces of electrodes. The resulting radical salts absorb visible light and, when oxidized, intend to take on the form of ions to dissociate per se. It was reported that such a redox system has a reversible nature (C. J. Shoot et al., Appl. Phys. Lett., 23 (2), 64, (1973)). By way of example, the inorganic thin films are discussed in the following description.
Diverse approaches have been suggested as to basic structure of ECD cells containing the electrolyte; the electrolyte is constituted by sulfuric acid gel (U.S. Pat. No. 3,708,220--M. D. Meyers, 1973; a counter electrode is established by EC material (WO3) and graphite, pigment is dispersed within the gel electrolyte and a background is given to a visual display while concealing the counter electrode (U.S. Pat. No. 3,819,252, R. D. Giglia, 1974); background material which gives the background to a visual display and conceals the counter electrode, is constituted by a porous film of pigment solidified with adhesive (U.S. Pat. No. 3,892,472, R. D. Giglia, 1975).
Although as stated above considerable effort has been devoted to the development of solid state thin-film ECD cells which utilize the electrolyte as a source of ion supply, the development of these ECD cells still faces many practical problems. The first of the problems concerns solubility of the WO3 film serving as the EC material into the electrolyte. If the sulfuric acid gel is employed as the ion supply source (as disclosed in the above referenced U.S. Pat. No. 3,708,220), then the evaporated WO3 thin film will be dissolved after being exposed to a high temperature atmosphere of about 80 deg. C for about 72 hours, thereby causing the destruction of the ECD cells. A successful solution of that problem was attempted through a solution of lithium perchloride into R-butyrolacetone or propylene carbonate (U.S. Pat. No. 3,704,057, L. C. Beagle, 1972) or ethylene glycol derivatives, for example, 2 ethoxy ethyl acetate as suggested by the inventors et al. A high temperature exposure test was conducted upon these electrolytes at 80 deg. C for 2 months. They were all found to be favorable to the electrolytes for ECD cells because of the fact that nothing was found through elemental analysis of tungsten element in these solutions.
The second problem is predicated upon a method of giving the background to a visual display. It is normal to use displays against a specific background. However, the combination of the graphite or a conductive coating and the EC material is not good for the background, although to this end, the electrolyte used should assume a specific opague color (the above referenced U.S. Pat. No. 3,819,252). One approach for providing the background is to form a gel of the electrolyte mixed with pigment such as TiO.sub.2 through the use of polyvinyl alcohol, etc. (The above referenced U.S. Pat. No. 3,819,252). However, the problem associated with the pigment containing electrolyte is that the pigment tends to coagulate and separate during preservation.
A film of pigment which is solidified with an adhesive while maintaining porosite, was suggested as an effective means for solving such a problem (U.S. Pat. No. 3,892,472). An example of a porous film is one containing a pigment solidified with an epoxy, while another example is a mixture of Teflon and a pigment squeezed to a thin film on a hot roll (this method is well known in the manufacture of a Teflon filter). Still another example is the paper making of pigment through an acrylic pulp. Since binders within these pigment containing porous films are organic compounds, they swell into the above discussed electrolyte solvents thereby destroying the porous film or weakening the mechanical strength.
The third and last problem concerns temperature resistance of the ECD cells. The ECD cells disclosed in the above patents are not able to tolerate expansion (an estimated ratio of about 10%) due to a temperature rise up to 50-100 deg. C during preservasion. This can be overcome by shortening a spacing between both electrodes, that is the display electrode and the counter electrode, and in other words reducing the volume of the electrolyte. However, provided that the spacing between both electrodes (thickness of the cells) is shortened, optical density of the pigment retaining background film will become smaller with the results that the shape of the counter electrode is caught by the viewer's eye therethrough. For example, the background film with the pigment content of 25 volume %, that is, the lowest limit capable of easily retaining the pigment, is not satisfactory as to opaqueness to serve as the background means unless its thickness is greater than 100.mu.. Meanwhile, an attempt to absorb expansion of the electrolyte due to a temperature rise of up to 50-100 deg. C only by "deflection" of a support plate (e.g. glass of about 1 mm thickness) is not successful because of a requirement that the ECD cell thickness be smaller than about 30 U. Further minimizing of the ECD cell thickness introduces a new problem of display electrode edge effects. Accordingly, it is an object of the present invention to provide cell structure for ECD cells capabling of providing an excellent background therefor and enhancing resistance against shocks caused by a temperature rise.